Pre-amplifier and mixer circuitry for a locator antenna

ABSTRACT

A pre-amplifier circuit for connection to an antenna of a human-portable locator includes a differential amplifier/mixer pair and means for allowing a common-mode “phantom” signal to modulate a transfer function of the differential amplifier/mixer pair. The common-mode phantom signal modulates the transfer function of the differential pre-amplifier “onboard” the antenna without the usual requirement for onboard power supply and signal oscillator. This technique uses the same electronic components to provide both pre-amplification and mixing functions, thereby improving circuit performance-to-cost ratio, reducing mixer power consumption, situating the necessary signal oscillator remotely from the mixer, and greatly improving the available system bandwidth by limiting spectral transmission demands to the mixed signal bandwidth alone.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.Utility patent application Ser. No. 12/367,419, entitled PRE-AMPLIFIERAND MIXER CIRCUITRY FOR A LOCATOR ANTENNA, filed Feb. 6, 2009 now U.SPat. No. 7,969,151, which claims priority to U.S. Provisional PatentApplication Ser. No. 61/027,212, entitled CONDUCTIVE BOBBINS AND MIXING,filed Feb. 8, 2008 and to U.S. Provisional Patent Application Ser. No.61/033,272, entitled PRE-AMPLIFIER AND MIXER CIRCUITRY FOR A LOCATORANTENNA, filed Mar. 3, 2008. This application is also related toco-pending U.S. patent application Ser. No. 11/248,539. The content ofeach of these applications is hereby incorporated by reference herein inits entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to portable locators that senseelectromagnetic emissions to locate buried utilities such as pipes andcables.

BACKGROUND

There are many situations where is it desirable to locate buriedutilities such as pipes and cables. For example, before starting any newconstruction that involves excavation, worker safety and projecteconomic concerns require the location and identification of existingunderground utilities such as underground power lines, gas lines, phonelines, fiber optic cable conduits, cable television (CATV) cables,sprinkler control wiring, water pipes, sewer pipes, etc., collectivelyand individually herein referred to as “buried objects.”

As used herein, the term “buried objects” includes objects locatedinside walls, between floors in multi-story buildings or cast intoconcrete slabs, for example, as well as objects disposed below thesurface of the ground. If excavation equipment such as a backhoe hits ahigh voltage line or a gas line, serious injury and property damage mayresult. Unintended severing of water mains and sewer lines generallyleads to messy and expensive cleanup efforts. The unintended destructionof power and data cables may seriously disrupt the comfort andconvenience of residents and bring huge financial costs to business.Therefore man-portable locators have been developed that senseelectromagnetic emissions to thereby locate buried utilities such aspipes and cables. This is sometimes referred to as “line tracing.” Ifthe buried cables or conductors carry their own electrical signal, theycan be traced by detecting the emissions at their appropriate frequency.Often signals with a known frequency are applied to pipes and cables viaa transmitter to enhance the ease and accuracy of the line tracing. Thiscan be done with an electrical clip in the case of a pipe, or with aninductive coupler in the case of a shielded conductor. Sometimes sondesare used to trace the location of pipes. These are tiny transmittersthat are inserted into a pipe and emit electromagnetic signals at adesired frequency.

Portable utility locators typically carry one or more antennas that areused to detect the electromagnetic signals emitted by buried pipes andcables, and sondes that have been inserted into pipes. The accuracy ofportable utility locators is limited by the sensitivity of theirantennas. Signal interference caused by capacitance or inductance withinthe antenna structures causes resonance and interference. Additionally,methods of processing signals detected by antennas in portable utilitylocators by amplifying them and mixing them, have traditionally sufferedfrom inefficiencies which include vulnerability to radio-frequencyinterference (RFI) and electromagnetic interference (EMI), and theintroduction of undesirable capacitance and inductance.

SUMMARY

The present disclosure relates generally to portable locators that senseelectromagnetic emissions to locate buried utilities such as pipes andcables.

In one aspect, the disclosure relates to an antenna interface circuitfor use in a portable locator comprising a switchable combinationpreamplifier and mixer circuit configured to be selectably switchableresponsive to a remotely generated phantom signal.

Various additional aspects, details, features, and functions aredescribed below with respected to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of various aspects of the presentinvention, reference is now made to the following detailed descriptionof the embodiments as illustrated in the accompanying drawings, in whichlike reference designations represent like features throughout theseveral views and wherein:

FIG. 1 is a perspective view of an antenna incorporating a conductivebobbin;

FIG. 2 is a schematic circuit diagram illustrating exemplary embodimentsof the pre-amplifier and mixer circuit of this invention suitable foruse with the antenna of FIG. 1 in a human-portable utility locator; and

FIG. 3 is a schematic circuit diagram of the pre-amplifier and mixercircuit embodiments of FIG. 2 illustrating exemplary circuit values.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention may be used to provide antennainterface circuitry that may be particularly suited for use with anantenna for detecting electromagnetic fields in a human-portable utilitylocator, such as that disclosed in co-pending U.S. patent applicationSer. No. 11/248,539 filed by Mark S. Olsson, et al. on Oct. 12, 2005 andentitled “Reconfigurable Portable Locator Employing Multiple SensorArray Having Flexible Nested Orthogonal Antennas,” the entire disclosureof which is incorporated herein by reference.

In accordance with one aspect, a pre-amplifier circuit for connection toan antenna of a man-portable locator includes an amplifier/mixer andcircuit that allows a common-mode phantom signal to modulate a transferfunction of the amplifier/mixer.

The circuitry may employ the repeatable temperature characteristics of asemiconductor p-n junction in the pre-amplifier to control thetemperature dependence of the amplifier/mixer, which opposes the effectsof the resistive temperature coefficient of the material in the antennaand thereby eliminates most temperature-dependent errors from the sensorarray output signals.

The circuitry may employ modulation that is remotely controlled by aphantom signal that can be turned ON or OFF to cause the system tooperate as a pre-amplifier or as a mixer, or as both, without componentor power supply duplication. Turning the phantom signal ON or OFF may beinterpreted as either applying or not applying said signal at the remoteend.

A preferred embodiment of the circuitry of this invention includesantenna pre-amplifier and mixer circuitry particularly suited for usewith each element of an antenna array employing any number of antennas,such as, for example, three orthogonal antennas each exemplified by theantenna 124 of FIG. 1 and having a generally cylindrical conductivebobbin on which antenna windings are supported to define a sensor axisnormal to the winding plane. For example, three of these antennas may benested in a concentric arrangement so that their respective sensor axesare mutually orthogonal to provide an antenna array for generatingsignals that represent measured signal strength and field angles inthree orthogonal dimensions. The pre-amplifier and mixer circuit of thisinvention are particularly suited for use with, for example, such anorthogonal antenna array.

Embodiments of the present invention may also provide a method ofmodulating the transfer function of each differential pre-amplifier usedin signal processing, which, in combination with a common mode phantomsignal using a single common oscillator that may be remotely disposedwith respect to the mixer, may provide greatly improved system bandwidthand immunity to electromagnetic interference (EMI) and radio frequencyinterference (RFI). Note that embodiments of circuits in accordance withaspects of the invention are suitable for use with any number ofantennas, and the discussion of configurations having three antennas ismerely an exemplary embodiment.

An individual common low-impedance pre-amplifier may be used for one ormore of the sub-coils in one antenna, and the present exemplaryembodiment uses a single common low-impedance pre-amplifier circuit forall sub-coils in each single antenna, requiring three pre-amplifiercircuits for the three antennas in the exemplary embodiment. Embodimentsof the present invention may also provide a method for managingpre-amplifier modulation to improve EMI and RFI immunity in the circuit.

FIG. 2 is a schematic circuit diagram illustrating a pre-amplifier andmixer circuit 202 and a processing and user interface system 204 of ahuman-portable locator employing an antenna array having three antennas(not shown) each exemplified by the antenna 124 (FIG. 1). The processingand user interface system 204 includes a power supply 206 connected toan oscillator 208 that is preferably connected to three substantiallyidentical circuit assemblies (not shown) each exemplified by thepre-amplifier and mixer circuit 202 for antenna 124. The oscillator 208is advantageously located in the processing and user interface system204 remote from the antenna 124 and the pre-amplifier and mixer circuit202.

Pre-amplifier load resistors 210 and 212 transfer current from powersupply 206 to the pre-amplifier and mixer circuit 202. A bandpass filter216 passes an analog baseband signal from pre-amplifier and mixercircuit 202 and serves to reject external EMI and RFI interference atthe input of an analog-to-digital (A/D) converter 218, which convertsthe analog baseband signal into a representative digital signal for useby the locator's digital processing circuitry (not shown). It shouldalso be noted that oscillator 208 in FIG. 2 may be externallysynchronized to local line frequency so that the line differencefrequencies are reduced to simply a DC offset which may be ignored bythe locator digital processing, and/or this may be combined with a pilotsignal so that the difference frequency is a specific offset from thetransmitted frequency and thus may be synchronously detected. Both ofthese options may be concurrently applied. Note further that while inthe preferred embodiment the multiple sub-coils of antenna 124 in FIG. 1drive a common base low-impedance amplifier, in an alternativeconfiguration a plurality of amplifiers or pre-amplifier circuits may beused.

The analog-to-digital converter is sampled exactly at power-linefrequency by using a separate power-line receiver; as a stronginterferer, it can be very simple. This receiver can drive a PLL thatprovides the analog-to-digital converter sampling clock. Note that fixedfrequencies close to the power-line frequency may also be employedbecause sampling at the power-line frequency produces a deep null at thepower-line frequency.

Any signal received by the locator system can be mixed to an arbitrarylow frequency because the system is adapted to provide mixing that maybe applied to all signals. For example, a 32,768 Hz signal that isreceived may be mixed in the manner described herein with a 32,770 Hzsignal to produce a 2 Hz signal that may be detected by theanalog-to-digital converter. Thus, in using the locator to trace powerlines, for example, the mixing function may be adapted to keep thedetection signal frequency lower than one-half the power-line frequency.If the analog-to-digital converter sampling rate is set to thepower-line frequency, then the Nyquist Sampling principle demonstratesthat no significant detection response can be found at the power-linefrequency (there is a response null at the sampling frequency). Thus,sampling at the power line frequency greatly improves locator systemperformance by making it immune to power-line interference. This methodalso removes power-line frequency harmonics as well because harmonicsnear the signal frequency are mixed down to some low frequency near DCand well below one-half of the power-line frequency. Because thebandwidth of the power-line interference signal is not zero, samplingclose to the power-line frequency (without locked to the exactpower-line frequency) retains most of the benefit of sampling at theexact power-line frequency and offers a significant benefit in theimplementation cost and effectiveness of a locating instrument.

As an example of this benefit, consider the need to detect 512 Hz Sondein the presence of a 60 Hz power-line interferer. The harmonics of 60 Hzin the vicinity of 512 Hz are 480 Hz (8th harmonic) and 540 Hz (9thharmonic). Assume mixing of 512 Hz down to 6 Hz using a 518 Hz mixersignal. Assume desired signal bandwidth to be 4 Hz (510 to 514 Hz), sothat the required detector frequency is 4 to 8 Hz. While sampling in thevicinity of the power line frequency (60 Hz), the analog-to-digitalconverter input is filtered from near DC (but not DC) to near but lessthan 30 Hz. An input pre-filter of 2 Hz to 18 Hz, for example,eliminates DC and very low frequency noise and 60 Hz aliases from theanalog-to-digital converter input, while passing the desired signal inthe range of 4 to 8 Hz. Note that the direct power-line harmonics of 480Hz and 540 Hz are also eliminated, assuming any reasonable line noisebandwidth.

The mixed-down power-line signal components include, for example, a 540Hz component mixed down by the 518 Hz mixer signal to 22 Hz and a 480 Hzcomponent similarly mixed down to 38 Hz. Assuming a power line signalbandwidth of 1 Hz, the resulting 21-23 Hz and 37-39 Hz signals are alsoremoved by the exemplary 2 Hz to 18 Hz pre-filter. Note that if themixer frequency varies slightly, or the line frequency varies slightly,or the Sonde frequency varies slightly, the locator system stilloperates correctly with these exemplary filter values. -Perhaps thestrongest interferers in this example are the 60 Hz and 180 Hzpower-line components, which are not only relatively distant from thedesired signal but are also filtered by the analog-to-digital converterand pre-filter. Even if the sampling occurs at 59 Hz, for example,instead of the precise 60 Hz power-line frequency, the exemplarypre-filter still attenuates the interference by 26 dB, a significantimprovement. Also note that removing DC (by using a lower pre-filterfrequency of 2 Hz in this example) removes the analog-to-digitalconversion effects of any unwanted DC offsets in the pre-amplifier ormixer.

Further in FIG. 2, the pre-amplifier and mixer circuit 202 in thisembodiment includes a differential amplifier/mixer transistor pair 232and 234 each disposed to receive a mixer signal at their respective basefrom either of bias resistors 220 and 222. Each mixer signal modulatesthe respective collector-emitter voltage drop in cooperation with therespective one of controlled current sinks 242 and 244. The antennaconnection 236 couples the antenna 124 to the differentialamplifier/mixer transistor pair 232 and 234. The pair of resistors 224and 228 with the bias capacitor 230 and transistor 226 together form abias network functioning as a Vbe multiplier to provide a constantvoltage at the collectors of transistor pair 232 and 234. The biascapacitor 230 provides a steady DC voltage supply to the two lowercontrolled current sinks 242 and 244 and operates to remove mixer signalcomponents. Resistors 238 and 240 each provide bias injection toregulate a respective one of the two controlled current sinks 242 and244 and also each complete one of the differential regeneration dividersformed on one side by the resistors 238 and 248 with the capacitors 246and 254 and on the other side by the resistors 240 and 250 with thecapacitors 252 and 260, substantially as shown. In the two differentialregeneration dividers, capacitors 246 and 252 each provide DC blockingfor regeneration in the respective sub-circuit, while resistors 248 and250 respectively set regenerative gain and couple with capacitors 246and 252 to an arbitrary low frequency. Capacitors 254 and 260 each actas the frequency determining point for regeneration in the respectivesub-circuit. In combination with impedance characteristics of antenna124, capacitors 254 and 260 establish a wide, flat responsecharacteristic. Resistors 256 and 258 act as balance resistors for thecurrent sink and establish the ratio of current between the V_(be)multiplier and the current sink. Such ratiometric control facilitatesoperation of these components as a band-gap regulator, providinginherent temperature compensation and tailoring temperature dependencein the pre-amplifier to exactly compensate for any temperature-relatedchanges in antenna coil resistance, thereby improving system temperatureindependence.

FIG. 3 provides a circuit diagram showing an exemplary embodiment of thecomponents of the pre-amplifier and mixer circuit 202 and their nominalvalues and may be appreciated with reference to the description of FIG.2. Note that Q13b, included for completeness, is not used in theillustrated circuit embodiment.

In FIG. 2, the pre-amplifier and mixer circuit 202 associated with eachof the antennas exemplified by the antenna 124 performs a mixing methodthat provides a common-mode phantom signal from oscillator 208 tomodulate the transfer function of the differential amplifier/mixertransistor pair 232 and 234. Instead of separating out or seeking toeliminate the modulation effects in the differential amplifier/mixerpair 232 and 234, these modulation effects are intentionally allowed andenhanced. The modulation is remotely controllable by a “phantom” signal(“phantom” in the sense that the signal is introduced remotely at the DCbias connector terminals to the collectors of differential transistorpair 232 and 234), which may be turned ON or OFF to cause the system tooperate either as a pre-amplifier or as a mixer. This provides componenteconomy by eliminating separate pre-amplifier and mixer circuits. Itfurther provides power economy by eliminating the power needed tooperate mixer circuit components because both pre-amplifier and mixeremploy the same electronic components. Thus, the pre-amplifier and mixercircuit 202 receives “phantom” power from a remote power supply and“phantom” modulation from a remote common mode oscillator 208 so thatneither need be provided onboard circuit 202. This method has thebenefit of improving the bandwidth of the system because thetransmission media need only support the differential mixed signal.

Each antenna exemplified by the antenna 124 is preferably terminateddirectly and locally to a dedicated pre-amplifier and mixer circuit 202to reduce circuit capacitance. Remote (“phantom”) power and modulation(e.g., from system 204) facilitates such localization by eliminating alllocal power and modulation requirements. Performing mixing andamplification functions locally in the antenna array 200 minimizesunwanted external pickup reduces EFI and RFI susceptibility. Because thesignal entering the transmission line between circuit 202 and system 204is already locally amplified (buffered), the signal-to-noise ratio (SNR)is improved. Further, with local mixing, this phantom method permitsfiltering to be introduced more effectively, without affecting thesystem's frequency coverage, because the output frequency spectrum tendsto be restricted. In the preferred embodiment, the mixing mode operatingin the pre-amplifier and mixer circuit 202 can be a sinusoidal or aswitched mode. Other waveforms, such as a complex waveform which sumstwo or more frequencies may be used to equal advantage. For instance,multiple frequencies might be applied to simultaneously mix severalfrequencies to a common frequency, or mix several frequencies to severalother frequencies, such as may be necessary in spread spectrumapplications. Note that spread spectrum and its related applications mayalso use pseudo-noise signals (PN or quasi-random) instead ofconventional sine-wave mixing sources. These and many other usefulsignal formats are accommodated by the structure described herein. Thedegree of modulation available during mixing is a function of thevarying signal level combined with the fixed or varying oscillatorlevel.

As a further example, the available frequency response characteristicmay be extended to lower frequencies through regeneration by providing acontrolled positive feedback with a loop gain less than unity. Suchfeedback lowers effective pre-amplifier circuit impedance, which in turnsignificantly improves low-frequency (LF) performance, lowering the LFcutoff frequency by as much as 60-dB. Adapting this technique to thecircuitry of this invention accommodates a wider frequency range withoutcompromising the antenna performance. The pre-amplifier and mixercircuit 202 can selectively activate regeneration to enhancelow-frequency search performance, effectively adding a filteringfunction at negligible cost and without additional power. Because lowfrequency enhancement results from a positive controlled feedback, anydirect control of the positive feedback by, for example, simplycontrolling the gain of the amplifier with remotely controlled loadimpedance (e.g., pre-amplifier load resistors 210 and 212 in FIG. 2)facilitates any desired response characteristic. The combination ofregeneration and analog-to-digital converter sampling at power linefrequencies is particularly advantageous, providing an improved lowfrequency performance without the usual limitations imposed by powerline harmonics. This feature is particularly useful when locating 16 Hzsondes, for example.

The antenna 124 and the pre-amplifier and mixer circuit 202 togetherprovide tailored temperature dependence in the preamplification tocompensate for temperature-induced changes in antenna coil resistance,thereby improving locator accuracy over a significant range of ambienttemperatures. This effect arises from employing the repeatabletemperature characteristics of the semiconductor p-n junction in thepre-amplification to control the temperature dependence of thedifferential amplifier/mixer pair 232 and 234, which opposes the effectsof the resistive temperature coefficient of the material in the antenna124. Before these teachings, the antenna material temperature dependencewas ignored as an error term, which limited the available systemaccuracy and utility until now.

In the preferred embodiment of the circuit of this invention, thetopology of the pre-amplifier and mixer circuit 202 is disposed toeliminate performance variation responsive to supply voltage changes.This benefit derives from the ratiometric amplification scheme of thisinvention, which ensures that DC characteristics, and therefore gain, donot fluctuate with supply voltage.

The mixing method of this invention for the first time provides acommon-mode “phantom” signal for use in modulating the transfer functionof the “onboard” differential pre-amplifier without the usualrequirement for onboard power supply and signal generation at theantenna, which increases cost and reduces performance for manywell-known reasons: eliminates components by removing the power supply,and local signal generating (oscillator) source, for example. Combiningthe phantom modulation with phantom signal operation provides for thefirst time a remotely-controlled phantom signal that allows onboardcircuit operation either as a simple pre-amplifier or as a mixer. Thistechnique uses the same electronic components to provide bothpreamplification and mixing functions, thereby improving circuitperformance-to-cost ratio, reducing mixer power consumption, situatingthe single necessary oscillator remotely from the mixer, facilitatingremote “phantom” circuit powering and “phantom” oscillator injection andgreatly improving the available system bandwidth by limiting spectraltransmission demands to the mixed signal bandwidth alone. Additionallythe circuit of this invention substantially eliminates the well-knownlocator antenna capacitance problem by terminating each antenna elementimmediately into a dedicated proximal mixer/pre-amplifier circuit.Finally, the circuitry of this invention yields more accurate linetracing performance.

Embodiments of the present invention may accomplish important functionsincluding 1) mixing and amplifying in the same hardware via remotecontrol; 2) remote controlled regeneration; 3) sampling of ananalog-to-digital converter in circuitry connected to the pre-amplifier;and 4) temperature compensation of the antenna via the pre-amplifier.The first and second functions are related by the fact that the load onthe pre-amplifier is remote to the pre-amplifier itself. Thus byapplying a common mode signal to each of the differential loadimpedances, the pre-amplifier becomes a mixer. However, the gain of anamplifier is also affected by its load impedance. Therefore, either thefirst function or the second function can be invoked remotely from thepre-amplifier at the load. The second and third functions are related bylow frequency extension. The “better” you make the low frequencyresponse, the “worse” the system performance becomes due to power linesignals. These pervade the environment of a man-portable buried utilitylocator. Therefore, if the circuit can sample the analog-to-digitalconverter (connected to the pre-amplifier) at the power line frequency,the circuit is more immune to this interference, and can thus the lowfrequency response of the locator can be extended using controlledregeneration. The circuit can only sample the analog-to-digitalconverter at the low frequency. Mixing higher frequencies down to afrequency that will pass through the narrow band analog-to-digitalconverter is therefore advantageous. Thus, the present invention has atriple beneficial effect. The fourth function occurs because thepre-amplifier is now local to the antenna and thus at relatively thesame temperature. Since the gain and operation of the pre-amplifier isunder control of the common mode DC biasing signal thereby importing thefirst and second functions, this also allows direct control oftemperature compensation as well.

The present invention provides a novel method useful in a man-portableburied utility locator that includes the steps of: receiving at a localpre-amplifier a common-mode phantom signal from a remote processing anduser interface system; adjusting a load impedance with the phantomsignal to provide positive feedback with a loop-gain less than unity inthe local pre-amplifier for the purpose of extending the low-frequencyresponse of the local pre-amplifier; connecting an output signal of thepre-amplifier to an analog-to-digital converter located distally in theremote processing and user interface system; and sampling theanalog-to-digital converter located distally in the remote processingand user interface system at a frequency substantially equivalent to apower line frequency for the purpose reducing the interference effectsof power line frequency upon the output of the analog-to-digitalconverter.

Clearly, other embodiments and modifications of this invention may occurreadily to those skilled in the art in view of these teachings.Therefore, the protection afforded this invention is to be limited onlyby the following claims, which include all such embodiments andmodifications when viewed in conjunction with the above specificationand accompanying drawings.

I claim:
 1. A portable locator comprising; a housing assembly; aprocessing and user interface sub-system disposed on or within thehousing assembly for providing information associated with a buriedobject; an antenna; and an antenna interface circuit coupled to theantenna and processing and user interface subsystem including aswitchable combination preamplifier and mixer circuit configured to beselectably switchable between preamplification and mixing responsive toa remotely generated phantom signal; wherein the antenna comprises aconductive bobbin and wherein the conductive bobbin comprises an elementof a nested omnidirectional antenna array.
 2. A portable locatorcomprising; a housing assembly; a processing and user interfacesub-system disposed on or within the housing assembly for providinginformation associated with a buried object; an antenna; and an antennainterface circuit coupled to the antenna and processing and userinterface subsystem including a switchable combination preamplifier andmixer circuit configured to be selectably switchable betweenpreamplification and mixing responsive to a remotely generated phantomsignal; wherein the antenna interface circuitry includes a semiconductorp-n junction to control temperature dependence of the combinationpreamplifier and mixer circuit.
 3. A portable locator comprising; ahousing assembly; a processing and user interface sub-system disposed onor within the housing assembly for providing information associated witha buried object; an antenna; and an antenna interface circuit coupled tothe antenna and processing and user interface subsystem including aswitchable combination preamplifier and mixer circuit configured to beselectably switchable between preamplification and mixing responsive toa remotely generated phantom signal; wherein the switchable combinationpreamplifier and mixer circuitry employs remotely controlled modulationthat is controlled by the phantom signal that can be turned to an onstate or an off state.
 4. The portable locator of claim 1, wherein thepreamplifier and mixer circuit are configured to be switchably operatedas both a preamplifier and a mixer.
 5. The portable locator of claim 1,further comprising an oscillator, wherein the oscillator is locatedremotely from the antenna and preamplifier and mixer circuit.
 6. Theportable locator of claim 5, wherein the oscillator is disposed remotelyfrom the antenna and preamplifier and mixer circuitry in the processingand user interface subsystem.
 7. A portable locator comprising; ahousing assembly; a processing and user interface sub-system disposed onor within the housing assembly for providing information associated witha buried object; an antenna; an antenna interface circuit coupled to theantenna and processing and user interface subsystem including aswitchable combination preamplifier and mixer circuit configured to beselectably switchable between preamplification and mixing responsive toa remotely generated phantom signal; and an oscillator, wherein theoscillator is located remotely from the antenna and preamplifier andmixer circuit and wherein the oscillator is externally synchronized to alocal line frequency.
 8. A portable locator comprising; a housingassembly; a processing and user interface sub-system disposed on orwithin the housing assembly for providing information associated with aburied object; an antenna; an antenna interface circuit coupled to theantenna and processing and user interface subsystem including aswitchable combination preamplifier and mixer circuit configured to beselectably switchable between preamplification and mixing responsive toa remotely generated phantom signal; and an oscillator; wherein theoscillator is located remotely from the antenna and preamplifier andmixer circuit and wherein the mixer frequency is selected such that amixed output frequency of a signal received from a buried object ismixed to a detection signal frequency lower than one half of a powerline frequency.
 9. The portable locator of claim 8, wherein the samplingrate of an analog-to-digital converter coupled to an output of theswitchable combination preamplifier and mixer circuit configured is setto substantially a power line frequency.